# std::atomic_ref<T>::fetch_sub

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< cpp‎ | atomic‎ | atomic ref

Provided only when `T` is an arithmetic type other than cv bool or a pointer-to-object type |  |
value_type fetch_sub( difference_type arg, std::memory_order order = std::memory_order_seq_cst ) const noexcept; |  | (constexpr since C++26)

Atomically replaces the current value of the referenced object with the result of arithmetic subtraction of the value and arg. This operation is a read-modify-write operation. Memory is affected according to the value of order.

- For signed integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.
- For floating-point types, the floating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not be conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior.
- For pointer types, the result may be an undefined address, but the operation otherwise has no undefined behavior.
  - If std::remove_pointer_t<T> is not a complete object type, the program is ill-formed.

This overload participates in overload resolution only if std::is_const_v<T> is false.

### Parameters

arg | - | the other argument of arithmetic subtraction
order | - | memory order constraints to enforce

### Return value

The value referenced by *`ptr`, immediately preceding the effects of this function.

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3508 (P3323R1) | C++20 | `fetch_sub` was meaningless for const T | constrained to accept only non-const `T`

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